A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                              C          ⁢                                          ⁢          D                =                              k            1                    *                      λ            NA                                              (        1        )            where λ, is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
The properties of EUV radiation require substantial changes in the nature of the optical system. In general, refractive optical elements would absorb too much of the wanted radiation, and so the optical systems for EUV illumination and projection are designed using reflective optical elements (mirrors). Furthermore, a gaseous atmosphere would absorb the EUV radiation, and so the optical system is housed in a vacuum chamber and operated substantially in a vacuum. These changes present many challenges in the design of optical systems for EUV radiation.
A significant part of the EUV light is absorbed in the mirrors due to the relatively low reflection of the mirrors; this causes the mirrors to heat up, and to heat up non-uniformly. This heat-up leads to mirror deformation and reduced optical performance. (In a lithographic apparatus, performance measures would be imaging & overlay.) In the projection optical system, mirrors are made with the low expansion coefficient materials. Even so, without active thermal conditioning of the mirrors, the deformation is too high to reach acceptable optical performance. In the examples to be described, cooling will be used as an example of thermal conditioning. The more general term “thermal conditioning” is used to encompass not only simple cooling, but any process for imposing a desired temperature distribution throughout the optical element. This could be by cooling, by heating or by application of different degrees of heating and/or cooling at different points in space and/or time. Where the description refers for example to a “cooling fluid”, “cooling plate” or “cooling body”, this is to be understood as merely an example of a thermal conditioning fluid, body, etc.
Thermally conditioning the mirrors directly with air or water channels tends to cause dynamic vibrations leading to unacceptable performance loss, at least in the most critical elements.